The invention relates to thermoplastic film structures that are sealable and peelable, and to methods of manufacturing the same.
A peelable film structure is described in publication WO 96/04178. The film structure comprises (a) a core layer comprising an olefin polymer and (b) a heat-sealable layer comprising a blend of low density polyethylene (LDPE) and a material incompatible with the LDPE, such as an olefin polymer or a co- or terpolymer of ethylene, propylene or butene. The film structure can be heat-sealed to a plastic container to form the lid of the container, or to itself to form a package. It is disclosed that when used with a plastic container, the film structure can be readily peeled from the container in order to open it.
U.S. Pat. No. 5,358,792 discloses a heat-sealable composition comprising (a) from about 30 to about 70 wt % of a low-melting polymer comprising a very low density ethylene-based copolymer defined by a density in the range of about 0.88 g/cm3 to about 0.915 g/cm3, a melt index in the range of about 1.5 dg/min to about 7.5 dg/min, a molecular weight distribution (Mw/Mn) no greater than about 3.5 and (b) from about 70 to about 30 wt % of a propylene-based polymer.
U.S. Pat. No. 5,443,915 discloses an oriented, polyolefin film that has a white-opaque cold seal-receptive skin layer on one side of a core layer and a vacuum deposited metal layer on the other side of the core layer. The cold seal-receptive polyolefin layer contains: (a) a slip agent in an amount sufficient to provide not more than about an 0.4 coefficient of friction to the surface of such layer; and (b) from about 10 to 40 wt % of titanium dioxide. The core layer is substantially free of opacifying filler and/or opacifying voids. The other side of the film is metallized to an optical density of at least 1.5, whereby the white pigment in the sealing layer in cooperation with the metallization gives a strong white-opaque appearance to the cold seal receptive layer.
U.S. Pat. No. 5,482,780 discloses an oriented polymeric α-olefin film having: an isotactic propylene homopolymer core; a cold seal release skin layer adherent to one side of the core; and a surface treated cold seal receptive layer or such treated layer with a cold seal cohesive composition over the surface treatment of said layer on the other side of the core. The cold seal release skin layer comprises a slip agent and a blend of two polymers, namely, an ethylene-propylene random copolymer containing about 2% to 8% of ethylene in such copolymer and an ethylene-butylene copolymer containing about 0.5% to 6% of ethylene in such copolymer. The cold seal receptive layer is of an ethylene-propylene random copolymer containing about 2 to 8% of ethylene.
U.S. Pat. No. 5,500,265 discloses a peelable film comprising (a) a core layer comprising an olefin polymer, (b) a skin layer on at least one surface of the core layer, the skin layer comprising: a blend of a butylene polymer with another olefin polymer or a polymer of butylene and at least one other olefin, and (c) a coating layer on the skin layer.
U.S. Pat. No. 5,716,698 relates to a peelable, oriented, opaque, multilayer polyolefin film comprising at least one opaque layer and a peelable top layer. All layers of the film are essentially vacuole-free. The opaque layer contains pigments having a mean particle diameter of from 0.01 to 1 μm.
U.S. Pat. No. 5,997,968 discloses a multilayer polyolefin film comprising at least three coextruded layers comprising an opaque base layer, an intermediate layer, and an outer peelable surface layer composed of two incompatible polyolefins, wherein the intermediate layer contains at least 80% by weight of a polyolefin having a lower melting point or lower glass transition temperature than the polyolefin forming the base layer.
U.S. Pat. No. 6,231,975 discloses a sealable film that comprises: (a) an inner layer comprising an olefin polymer; (b) a sealing layer; and (c) a separable layer positioned between the inner layer and the sealing layer, the separable layer comprising (1) ethylene-propylene block copolymer or (2) a blend of polyethylene and another olefin which is incompatible with the polyethylene, specifically either (i) polypropylene homopolymer or (ii) ethylene-propylene block copolymer.
U.S. Pat. No. 6,248,442 relates to multilayer films that are heat-sealable over a broad temperature range. The '442 patent also relates to multilayer films that provide easy-opening and hermetic seals to packages. The films of the '442 patent are made up of a core layer comprising linear low density polyethylene (LLDPE) and at least one skin layer having a melting point of at least 10° C. below the core layer melting point.
There is still room for improvement, however, in the field of sealable and peelable film structures. In particular, a need still exists for a film structure that maximizes the advantageous combination of (i) a sealable outer layer that seals well to itself or other surfaces over a broad temperature range and (ii) a particular multilayer design that permits (a) separation of the sealed outer layer from at least one other layer of the film and (b) the seal to be readily opened without creating a “z-direction” tear.
A z-direction tear disrupts the integrity of a multilayer film when the film is pulled apart at the seal. A film with a z-direction tear has not simply separated at the seal line. Instead, the separation, or tear, has extended to other layers of the film. It is difficult to properly reclose a package that has a z-direction tear, thereby hampering the ability of the package to maintain the freshness of its contents. When z-direction tears can be eliminated, packages are easily refolded and sealed by simple mechanical means, such as a clip.